1. Field of the Invention
Embodiments of the invention relate to a digital double sampling method, a related complementary metal oxide semiconductor (CMOS) image sensor, and a digital camera comprising the CMOS image sensor. In particular, embodiments of the invention relate to a digital double sampling method comprising inverting digital data, a related CMOS image sensor, and a digital camera comprising the CMOS image sensor.
2. Description of the Related Art
CMOS image sensors, which are widely used in digital cameras, convert optical signals into electrical signals. An operation for converting an optical signal into an electrical signal is performed by a pixel of the CMOS image sensor, wherein the pixel includes a photodiode and a read-out circuit. The photodiode generates electric charges using absorbed light, converts the generated electric charges into an analog current, and delivers the analog current to the read-out circuit. The read-out circuit converts the analog signal into a digital signal and outputs the digital signal.
In an analog-to-digital conversion process, a comparator receives an analog voltage and compares the analog voltage with a ramp voltage. The comparator compares the analog voltage with the ramp voltage, and uses a counter to count until the ramp voltage is greater than an analog voltage. Once the counter stops counting, a count value is digital data corresponding to an analog voltage, that is, the count value is the digital data into which the analog voltage has been converted.
An up-down counter is typically used to perform digital double sampling (DDS). DDS means obtaining a difference (Dsig-Drst) between digital data Drst obtained by converting a first analog signal output by an initialized pixel into digital data, and digital data Dsig obtained by converting a second analog signal received from a pixel that has received an external image signal into digital data, wherein the second analog signal corresponds to an external image signal.
Because the up-down counter performs up-count and down-count functions, the up-down counter has a relatively large number of gates. Therefore, the up-down counter occupies a relatively large area. In addition, power consumption is limited in a high resolution image sensor.